We have created an electrically controlled phonon-driven thermal switch – Congrats Brian Foley!

We have recently demonstrated the ability to control phonon scattering rates at room temperature via the application of an electric field in lead zirconate titanate (PZT) thin films.  Upon application of the electric field, the ferroelastic domains reconfigure and influence the thermal conductivity with sub-second switching times.  This is the first demonstration of an active switch of phonon thermal conductivity at room temperature.  This work was recently published in Nano Letters (Nano Letters 15, 1791-1795 (2015)) – congrats Brian Foley on your exciting results and research that formed the core of this discovery.  This work was in collaboration with Dr. Jon Ihlefeld from Sandia National Laboratories.


Dynamic control of thermal transport in solid-state systems is a transformative capability with the promise to propel technologies including phononic logic, thermal management, and energy harvesting. A solid-state solution to rapidly manipulate phonons has escaped the scientific community. We demonstrate active and reversible tuning of thermal conductivity by manipulating the nanoscale ferroelastic domain structure of a Pb(Zr0.3Ti0.7)O3 film with applied electric fields. With subsecond response times, the room-temperature thermal conductivity was modulated by 11%.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories, the Air Force Office of Scientific Research (FA9550-13-1-0067), and the National Science Foundation (CBET-1339436). Sandia National Laboratories is a multipro- gram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04−94AL85000. The authors wish to acknowledge the technical assistance of Mia Blea-Kirby, Garry Bryant, Benjamin Griffin, and John T. Gaskins. Critical review of this manuscript by Paul G. Clem, Thomas E. Beechem, and Jon-Paul Maria is greatly appreciated.

Thermal conductivity of annealed GaN surfaces – Congrats Chet!

Chet Szwejkowski’s paper, “Size effects in the thermal conductivity of gallium oxide (beta-Ga2O3) films grown via open-atmosphere annealing of gallium nitride (GaN),” was recently published in Journal of Applied Physics (J. Appl. Phys. 117, 084308 (2015)). In this work, we demonstrated that the thermal conductivity of Beta-phase Ga2O3 that forms on the surface of GaN while annealing in open atmosphere is polycrystalline and has a thermal conductivity of 8.8 ± 3.4 W m−1 K−1. This relatively high thermal conductivity has major implications for annealing during the creation of GaN contacts.  Congrats Chet!!!



Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devicesdue to of its unique electrical properties: a wide band gap, high breakdown field, and high electron mobility. However, thermal management has become a limiting factor regarding efficiency, lifetime, and advancement of GaN devices and GaN-based applications. In this work, we study the thermal conductivity of beta-phase gallium oxide (-GaO) thin films, a component of typical gate oxides used in such devices. We use time domain thermoreflectance to measure the thermal conductivity of a variety of polycrystalline -GaO films of different thicknesses grown via open atmosphere annealing of the surfaces of GaN films on sapphire substrates. We show that the measured effective thermal conductivity of these -GaO films can span 1.5 orders of magnitude, increasing with an increased film thickness, which is indicative of the relatively large intrinsic thermal conductivity of the -GaO grown via this technique (8.8 ± 3.4 W m−1 K−1) and large mean free paths compared to typical gate dielectrics commonly used in GaN device contacts. By conducting time domain thermoreflectance (TDTR) measurements with different metal transducers (Al, Au, and Au with a Ti wetting layer), we attribute this variation in effective thermal conductivity to a combination of size effects in the -GaO film resulting from phonon scattering at the -GaO/GaN interface and thermal transport across the -GaO/GaN interface. The measured thermal properties of open atmosphere-grown -GaO and its interface with GaN set the stage for thermal engineering of gate contacts in high frequency GaN-based devices.


The material is based upon work partially supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0067 (Subaward No. 5010-UV-AFOSR-0067), the National Science Foundation (CBET-1339436) and the Commonwealth Research Commercialization Fund (CRCF) of Virginia.

Electron-phonon coupling in Au depends on electron and phonon temperatures: Congrats Ash!

Ash Giri’s paper, “Experimental evidence of excited electron number density and temperature effects on electron-phonon coupling in gold films,” was recently published in Journal of Applied Physics (J. Appl. Phys. 117, 044305 (2015)). In this work, we studied the electron-phonon coupling processes in Au films after short pulsed laser heating using a range of laser fluences to control electron temperature while varying the lattice temperature via a LN2-cooled cryostat.  We provide quantitative experimental evidence of the transition from nonequilibrium-to-equilibrium dominated electron-phonon coupling relaxation in thin films. We show that the temperature dependence of the electron-phonon coupling factor in Au varies based on both the lattice and electron temperatures, in contrast to the two temperature model. This paper focuses on answering the fundamental question of scattering mechanisms and their relation to the experimental conditions that lead to different energy relaxation rates between the electronic and vibrational states in metals. In doing so, our measurements provide experimental evidence that elucidate novel transport processes in thin metal films along with resolving the discrepancies in nearly 20 years of experimental data.  Congrats Ash!!!


The electronic transport properties of metals with weak electron-phonon coupling can be influenced by non-thermal electrons. Relaxation processes involving non-thermal electrons competing with the thermalized electron system have led to inconsistencies in the understanding of how electrons scatter and relax with the less energetic lattice. Recent theoretical and computational works have shown that the rate of energy relaxation with the metallic lattice will change depending on the thermalization state of the electrons. Even though 20 years of experimental works have focused on understanding and isolating these electronic relaxation mechanisms with short pulsed irradiation, discrepancies between these existing works have not clearly answered the fundamental question of the competing effects between non-thermal and thermal electrons losing energy to the lattice. In this work, we demonstrate the ability to measure the electron relaxation for varying degrees of both electron-electron and electron-phonon thermalization. This series of measurements of electronic relaxation over a predicted effective electron temperature range up to 􏰤3500 K and minimum lattice temperatures of 77 K validate recent computational and theoretical works that theorize how a nonequilibrium distribution of electrons transfers energy to the lattice. Utilizing this wide temperature range during pump-probe measurements of electron-phonon relaxation, we explain discrepancies in the past two decades of literature of electronic relaxation rates. We experimentally demonstrate that the electron-phonon coupling factor in gold increases with increasing lattice tem- perature and laser fluences. Specifically, we show that at low laser fluences corresponding to small electron perturbations, energy relaxation between electrons and phonons is mainly governed by non-thermal electrons, while at higher laser fluences, non-thermal electron scattering with the lattice is less influential on the energy relaxation mechanisms.


This material is based upon work supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-13-1-0067 (P.E.H—AFOSR Young Investigator Program).

Congrats Ramez Cheaito on Paper published in Phys. Rev. B focusing on thermal boundary conductance accumulation – measurements and theory

Ramez Cheaito’s paper, “Thermal boundary conductance accumulation and interfacial phonon transmission across interfaces: measurements and theory,” has recently been published in Physical Review B (Phys. Rev. B 91, 035423 (2015)).  In this work, we develop the analytical formalism to mathematically model the accumulation of phonon energy across interfaces, and how this accumulation contributes the thermal boundary conductance and interfacial phonon transmission.  For example, this formalism gives the ability to determine what percentage of the phonon spectrum in a material adjacent to the interface contributes to a certain % of the thermal boundary conductance.  This metric is immensely important for thermal transport in nanosystems.  We validate our theory with a wide array of measurements across metal/nonmetal interfaces via TDTR measurements of thermal boundary conductance.  Congrats Ramez!!!


The advances in phonon spectroscopy in homogeneous solids have unveiled extremely useful physics regarding the contribution of phonon energies and mean-free paths to the thermal transport in solids. However, as material systems decrease to length scales less than the phonon mean-free paths, thermal transport can become much more impacted by scattering and transmission across interfaces between two materials than the intrinsic relaxation in the homogeneous solid. To elucidate the fundamental interactions driving this thermally limiting interfacial phonon scattering process, we analytically derive and experimentally measure a thermal boundary conductance accumulation function. We develop a semiclassical theory to calculate the thermal boundary conductance accumulation function across interfaces using the diffuse mismatch model, and validate this derivation by measuring the interface conductance between eight different metals on native oxide/silicon substrates and four different metals on sapphire substrates. Measurements were performed at room temperature using time-domain thermoreflectance and represent the first-reported values for interface conductance across several metal/native oxide/silicon and metal/sapphire interfaces. The various metal films provide a variable bandwidth of phonons incident on the metal/substrate interface. This method of varying phonons’ cutoff frequency in the film while keeping the same substrate allows us to mimic the accumulation of thermal boundary conductance and thus provides a direct method to experimentally validate our theory. We show that the accumulation function can be written as the product of a weighted average of the interfacial phonon transmission function and the accumulation of the temperature derivative of the phonon flux incident on the interface; this provides the framework to extract an average, spectrally dependent phonon transmissivity from a series of thermal boundary conductance measurements. Our approach provides a platform for analyzing the spectral phononic contribution to interfacial thermal transport in our experimentally measured data of metal/substrate thermal boundary conductance. Based on the assumptions made in this work and the measurement results on different metals on native oxide/silicon and sapphire substrates, we demonstrate that high-frequency phonons dictate the transport across metal/Si interfaces, especially in low Debye temperature metals with low-cutoff frequencies.

Our work at U.Va. was supported through the Office of Naval Research, Young Investigator Program (Grant# N00014-13-4-0528).

Thermal boundary resistance at metal/GaN interfaces can be significant: Congrats Brian Donovan

Brian Donovan’s paper, “Thermal boundary conductance across metal-gallium nitride (GaN) interfaces from 80 – 450 K,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 203502 (2014)).    In this work, we show that the thermal boundary conductance across metal/GaN interfaces can impose a thermal resistance similar to that of GaN/substrate interfaces. We also show that these thermal resistances decrease with increasing operating temperature and can be greatly affected by inclusion of a thin adhesion layers.  Congrats Brian!!!


Thermal boundary conductance is of critical importance to gallium nitride (GaN)-based device performance. While the GaN-substrate interface has been well studied, insufficient attention has been paid to the metal contacts in the device. In this work, we measure the thermal boundaryconductance across interfaces of Au, Al, and Au-Ti contact layers and GaN. We show that in these basic systems, metal-GaN interfaces can impose a thermal resistance similar to that of GaN-substrate interfaces. We also show that these thermal resistances decrease with increasing operating temperature and can be greatly affected by inclusion of a thin adhesion layers.

The material is based upon the work partially supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0067 (Subaward No. 5010-UV-AFOSR-0067) and the National Science Foundation (CBET-1339436). This work was partially supported by the Commonwealth Research Commercialization Fund (CRCF) of Virginia. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

Glass-like thermal conductivity of crystalline strontium niobate; a crystalline solid synthesized with solution chemistry: Congrats Brian Foley!

Brian Foley’s paper, “Glass-like thermal conductivity of (010)-textured lanthanum-doped strontium niobate synthesized with wet chemical deposition,” has been published in the Journal of the American Ceramic Society (DOI: 10.1111/jace.13318).    In this work, we have demonstrated that solution-based chemistry can produce highly textured, single crystalline oxide-films of strontium niobate with ultra-low thermal conductivities.  the low thermal conductivities originate due to the variable and layered bonding environments in this perovskite structure.  Congrats Brian!!!


We have measured the cross-plane thermal conductivity (κ) of (010)-textured, undoped, and lanthanum-doped strontium niobate (Sr2−xLaxNb2O7−δ) thin films via time-domain thermoreflectance. The thin films were deposited on (001)-oriented SrTiO3 substrates via the highly-scalable technique of chemical solution deposition. We find that both film thickness and lanthanum doping have little effect on κ, suggesting that there is a more dominant phonon scattering mechanism present in the system; namely the weak interlayer-bonding along theb-axis in the Sr2Nb2O7 parent structure. Furthermore, we compare our experimental results with two variations of the minimum-limit model for κ and discuss the nature of transport in material systems with weakly-bonded layers. The low cross-plane κ of these scalably-fabricated films is comparable to that of similarly layered niobate structures grown epitaxially.

B.M.F. is grateful for support from the ARCS Foundation Metro Washington Chapter. P.E.H. is appreciative for funding through the Army Research Office (W911NF-13-1-0378) and the NSF EAGER program (CBET-1339436). This work was performed in part at the Center for Atomic, Molecular, and Optical Science (CAMOS) at the University of Virginia. This work was supported, in part, by the Laboratory Directed Research and Development (LDRD) pro- gram at Sandia National Laboratories (H.B-S., M.J.C., D.L.M., P.G.C, J.F.I., P.E.H.). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. This project was supported by Financial Assistance Award No. 01-79-14214, awarded by U.S. Department of Commerce Economic Development Adminis- tration, to the University of Virginia (P.E.H, B.M.F). The content is solely the responsibility of the authors and does not necessarily represent the official views of the U.S. Department of Commerce Economic Development Administration.

Low thermal conductivity of nano-grained BaTiO3 – Congrats Brian Donovan!

Brian’s paper, “Spectral phonon scattering effects on the thermal conductivity of nano-grained barium titanate,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 082907 (2014)).  In this work, we show that grain sizes as large as 50 nm can still lead to reductions in the thermal conductivity of BaTiO3 thin films.  For films with grain sizes of 36 nm (on average), the thermal conductivity of the BaTiO3 thin film can reach as low as 1.0 W/m/K.  We attribute this reduction in thermal conductivity to the spectral nature of phonon mean free paths in BaTiO3, with some phonons having mean free paths larger than the grain sizes (which, in our samples, range from 36-63 nm).  This is in contrast to the typically assumed view of phonon transport in ABO3 perovskite ferroelectrics in which gray phonon mean free paths are assumed with length scales less than 5 nm.  Congrats Brian!!!


We study the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K. We show that the thermal conductivity of BariumTitanate (BaTiO) decreases with decreasing grain size as a result of increased phononscattering from grain boundaries. We analyze our results with a model for thermal conductivitythat incorporates a spectrum of mean free paths in BaTiO. In contrast to the common gray mean free path assumption, our findings suggest that the thermal conductivity of complex oxide perovskites is driven by a spectrum of phonons with varying mean free paths.

We appreciate funding from the National Science Foundation (CBET-1339436), the Army Research Office, Grant No. W911NF-13-1-0378, and the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0067 (Subaward No. 5010-UV-AFOSR-0067). This work was also supported, in part, by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories.